This relates generally to electronic devices, and more particularly, to electronic devices with displays.
Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user.
Liquid crystal displays contain a layer of liquid crystal material. Display pixels in a liquid crystal display contain thin-film transistors and electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in a display pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of the display pixel.
Substrate layers such as color filter layers and thin-film transistor layers are used in liquid crystal displays. The thin-film transistor layer contains an array of the thin-film transistors that are used in controlling electric fields in the liquid crystal layer. The color filter layer contains an array of color filter elements such as red, blue, and green elements. The color filter layer provides the display with the ability to display color images.
In an assembled display, the layer of liquid crystal material is sandwiched between the thin-film transistor layer and the color filter layer. Polyimide passivation layers cover the inner surface of the color filter layer and the upper surface of the thin-film transistor layer. An array of column spacers is formed on the inner surface of the color filter layer to maintain a desired gap between the color filter layer and the thin-film transistor layer. Column spacers are typically formed from hard organic materials such as photoresist.
There are typically two types of column spacers in a liquid crystal display. A relatively sparse set of main column spacers extends between the color filter layer and the thin-film transistor layer. The thickness of the column spacers and their associated landing pads establishes the amount of separation between the color filter layer and the thin-film transistor layer. Another set of column spacers, referred to as subspacers, has structures that extend only partway between the color filter layer and the thin-film transistor layer. Subspacers are used to prevent the thin-film transistor layer and column spacer from contacting one another. The subspacers do not extend all the way between the color filter layer and thin-film transistor layer to accommodate deformation of the color filter layer relative to the thin-film transistor upon a drop in ambient temperature for the display.
There are tradeoffs involved when determining an appropriate number column spacers to use in a given display. If too few of the main column spacers are provided, there will be insufficient support for the display. This will make the display susceptible to an undesirable visual effect called pooling mura. If too many of the main column spacers are provided, the display will become overly stiff. This will make the display prone to stress-induced birefringence when deformed, leading to undesired light leakage effects. With existing column spacer designs, it can be challenging to identify an acceptable tradeoff between pooling and light leakage. Displays are often sensitive to manufacturing variations and may exhibit more pooling and light leakage effects than desired.
It would therefore be desirable to be able to provide a display with an improved column spacer configuration.